This invention concerns the production of unsaturated compounds from chlorinated compounds using phase transfer catalysts in combination with a base.
Various methods are known to dehydrohalogenate halogenated hydrocarbons using a base and a phase transfer catalyst. For instance, U.S. Pat. No. 3,664,966 describes such a process wherein an aqueous base in conjunction with an organic quaternary salt. In the examples, 1,1-dichloroethylene (vinylidene chloride) is produced from 1,1,2-trichloroethane. The advantages of the phase transfer catalyst is an increase in reaction rate while allowing the reaction to be conducted at lower temperatures.
A similar process is described in U.S. Pat. No. 3,754,044. In this patent, a quaternary ammonium compound and a phosphate ester are utilized in combination as the phase transfer catalyst system. The phosphate ester is taught to act as a promoter.
While the processes described above produce enhanced reaction rates, such methods have generally not been used on a commercial scale because the phase transfer catalysts are expensive and lost during the process. Thus, such processes do not produce the desired products in a cost effective manner.
There followed U.S. Pat. No. 4,418,232 which describes an improved dehydrohalogenation process wherein a phase transfer catalyst is used. In this process, a cascade arrangement of reactors is described such that partially spent alkali solution from the first reactor is recycled back to a stage reactor prior to the primary reactor where fresh caustic solution is introduced. While this process recycles the caustic, the phase transfer catalyst is in large part not recycled because the phase transfer catalysts have greater solubility in the organic phase than in the aqueous phase.
More recently, U.S. Pat. No. 4,605,800 describes an improved dehydrohalogenation process to produce chloroprene from 3,4-dichlorobutene-1 using a phase transfer catalyst and caustic. In this process, the reactor effluent is sent to a decanter where the organic phase is separated from the aqueous phase. The organic phase is then sent to a steam-stripper whereby product is recovered and the heels from the stripper are returned to the reactor. However, since contaminants are present in the heels which would build-up if continuously recycled, from 2 to 20 percent of the heels are purged to prevent contaminant build-up. Moreover, it is know that phase transfer catalysts in general will decompose to some extent at temperatures greater than 130.degree. C. When this is done, catalyst is also purged. In addition, catalyst is lost in the decanter as well as the steam-stripper. Hence, make up catalyst must be added to the heels to maintain the proper amount of catalyst in the reaction.
It is apparent that new and improved processes are desirable in dehydrohalogenation processes using phase transfer catalyst to provide more cost effective methods of production.